Millions of years ago, when dinosaurs roamed the earth and our mammalian ancestors swung from the vines, no one worried about their hair.

It may still be a jungle out there and a constant struggle to survive, but some things have changed. No longer can you get by with just opposable thumbs: now you also need sexy hair.

"It's a life or death thing," says veteran hair sty;ist Jessica L. Baer of Diego's at the Loft. Jesse, who herself sports a most stylish 'do--dyed a streaked blond and chopped at ear level with whimsical little sideburns--says her clients tell her that "it's like they're going to die if they don't get the right haircut."

Hopefully it won't come to that. But just in case, let's take a quick look at what's hot and what's not this fall, according to Jesse, Edward J. Shields and Dale A. Julio of DHR Stylists and Jerry E. Lauretano of Jerry's Underground. Hot:

WIGS, WIGS, WIGS!

"Any kind of fake hair--that's the hottest thing right now," says Jesse, whose hair, paradoxically, is real.

The upsurge of prosthetic hair will come as a pleasant surprise to all those victims of male-pattern balding out there--you know who you are! And until now, so did everyone else.

But thanks to the frenetic pace of le monde de la mode, you can now don your toupee and voila! Tres chic!

It's just as well, because according to Edward of DHR Stylists, a balding man is a cheerless man.

"Especially if their hair is thinning, it becomes the keystone of their existence," he says, with a sagacity born of 15 years in the hair styling business. "There's no one more hair conscious than a man with thinning hair."

RETRO '60 s

A word of caution: "the '60s involves flat sides, puffiness on top and something called "tucks" in the vicinity of the ear, and should be part of a platform shoes and bellbottoms ensemble.

A word of caution: "the '60s thing," as Jesse calls it, is rapidly becoming passe. Pas du tout dans le vent. Jesse, clad in a black velvet body suit and calf-high boots, probably knows what she's talking about.

FOR GALS: SHORT HAIR

"On my women clientele--thank God they're going shorter," Jerry tells us. Thank God indeed! Long hair may work for Rapunzel and her ilk, but here in Harvard Square it's Hare Krishnas and too-busy-to-blowdry undergrads all the way.